Field of the Invention
The present disclosure relates to an image forming apparatus which performs development by adhering a toner particle to a latent image and an image forming method.
Description of the Related Art
Conventionally, an image forming apparatus which performs image formation on a desired sheet is known. To perform the image formation, various methods are proposed. One of the known methods is charging a toner particle and using an electrostatic force to perform the image formation. With an image forming apparatus of an electrophotographic apparatus type employing this method, when a charge amount of the toner particle (or toner particle charge amount) changes, density and quality of an output image accordingly change. The toner particle charge amount changes in accordance with various conditions such as a use environment, the density of the output image and output elapsed time. Thereby, if no control to stabilize the output is performed, the output image varies in accordance with the condition.
Further, an electrophotographic type image forming method using a two-component developing device, or a method to perform the image formation using the toner particle and a carrier particle as a developer is also known. With the image forming apparatus employing this method, a toner consumption amount is predicted from the image data. Then, the image forming apparatus supplies the toner which is almost the same amount with the toner consumption amount as predicted. It is also known to control or adjust the toner supply amount using an output value of an inductance sensor and the like which measures a density of the toner particle in the developer from a difference in magnetic permeability between the toner particle and the carrier particle in the developer.
In the two-component developing device, generally, the toner particle charge amount changes depending on a mixing ratio of the toner particle and the carrier particle in the developing device. As the ratio of the toner particle reduces, the toner particle charge amount increases. In a case where the toner particle charge amount increases, the toner particle adhering to a constant charged latent image reduces. On the contrary, in a case where the toner particle charge amount reduces, the toner particle adhering to the constant charged latent image increases.
Thereby, it is possible to stabilize the toner particle charge amount and the density of the output image by adjusting the toner particle supply amount and changing the mixing ratio of the toner particle and the carrier particle in the developing device. To this end, conventionally, a patch image for output density measurement is output. Then, patch density and the toner amount are obtained on an image carrier, on a transfer body and the like. Feedback control, through which the toner supply amount is controlled so that the output density matches with target density based on the obtained patch density and the toner amount, is well known. Through the control, in addition to the toner supply amount in accordance with the image data or the supply amount adjustment by the inductance sensor, the toner is also supplied based on the adjusted toner supply amount calculated based on the density of the output patch image. As a result, the toner charge amount and the toner density can be adjusted.
A control mechanism for stabilizing the density of the output image by adjusting the toner supply amount based on the output result of the patch image is the feedback control through which various adjustments are performed after measuring the patch density or the toner amount. Thereby, in principle, a delay is caused in control. Further, it takes time before the toner particle charge amount changes by following the toner supply adjustment so that delay is inevitably caused in control, which causes density deviation in a short period. To solve such problem, Japanese Patent Application Publication Laid-Open No. 2001-42613 discloses a technology to perform feed forward control through which, to stabilize the image density, the toner particle charge amount is estimated and a contrast potential in the image formation is restrained in real time.
In the feed forward control as mentioned, however, it sometimes causes a problem in that it is not possible to sufficiently suppress an influence due to the change of the toner particle charge amount. In an electrophotographic technology, the image is formed using electrostatic force. Thereby, it is desired that the toner particle charge amount remains as unchanged as possible. However, in a case where the contrast potential in the image formation is adjusted based on a prediction of the toner particle charge amount, regardless of a value of the toner particle charge amount, an adjustment is performed so that a toner developing amount to the image carrier is maintained constant. As a result, the toner particle charge amount remains different from the toner particle charge amount which corresponds to intended image density in value. Thereby, in a subsequent step, in a case where there is a step which gives influence on a transfer property in the image formation due to the variation in the toner particle charge amount, the image formation may not properly be performed.
Further, in a transfer step, in a case where the toner particle charge amount is different from the charge amount corresponding to the intended image density in value, to perform a proper transfer, the toner particle charge amount is insufficient or too much. As a result, the transfer property is changed so that the image density or quality is deteriorated. In particular, this is largely influenced when using a secondary color/tertiary color on which toner of one or more colors are overlapped.